runtime: sample large heap allocations correctly

Remove an unnecessary check on the heap sampling code that forced sampling
of all heap allocations larger than the sampling rate. This need to follow
a poisson process so that they can be correctly unsampled. Maintain a check
for MemProfileRate==1 to provide a mechanism for full sampling, as
documented in https://golang.org/pkg/runtime/#pkg-variables.

Additional testing for this change is on cl/129117.

Fixes #26618

Change-Id: I7802bde2afc655cf42cffac34af9bafeb3361957
GitHub-Last-Rev: 471f747af845395d458096bea26daa93b91120be
GitHub-Pull-Request: golang/go#29791
Reviewed-on: https://go-review.googlesource.com/c/158337
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/runtime/malloc.go

@@ -1012,7 +1012,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	}
 
 	if rate := MemProfileRate; rate > 0 {
-		if size < uintptr(rate) && int32(size) < c.next_sample {
+		if rate != 1 && int32(size) < c.next_sample {
 			c.next_sample -= int32(size)
 		} else {
 			mp := acquirem()

src/runtime/testdata/testprog/memprof.go

@@ -21,7 +21,10 @@ var memProfBuf bytes.Buffer
 var memProfStr string
 
 func MemProf() {
-	for i := 0; i < 1000; i++ {
+	// Force heap sampling for determinism.
+	runtime.MemProfileRate = 1
+
+	for i := 0; i < 10; i++ {
 		fmt.Fprintf(&memProfBuf, "%*d\n", i, i)
 	}
 	memProfStr = memProfBuf.String()